St Mark's Fly

Bibio marci


The St Mark's Fly is a very common, long, shiny black fly which can be found in large numbers during the spring around woodland edges, fields and wetlands. It hangs in the air over the vegetation, drifting along with its legs dangling in underneath it. St Mark's Flies will often land on any objects in their way, including fence posts and people, and are rather sluggish at rest. The larvae live in the soil feeding on roots, grasses and rotting vegetation, and are often found around compost heaps. The adults feed on nectar and are considered as important pollinators for fruit trees and other plants.

How to identify

St Mark's Flies are black and shiny; males and females look very different, however: the male has very large eyes, while the female has a small head with tiny eyes. There are several species of closely related Bibio flies that can be very difficult to tell apart. In the late summer, the Heather Fly (Bibio pomonae) hatches in large number on moorlands; it looks very similar to the St Mark's Fly, but the base of its legs is orangey-red. Another species is Bibio hortulanus, common in gardens in spring; in this species, the female is not black but a rich brick-red.

Where to find it



When to find it

  • April
  • May

How can people help

Many of our commonly overlooked insects are important pollinators for all kinds of plants, including those which we rely on like fruit trees. The Wildlife Trusts recognise the importance of healthy habitats to support all kinds of species throughout the food chain, so look after many nature reserves for the benefit of wildlife. You can help too: volunteer for your local Wildlife Trust and you could be involved in everything from coppicing to craft-making, to stockwatching to surveying.

Species information

Common name
St Mark's Fly
Latin name
Bibio marci
Body length: 1.1cm
Conservation status